This invention is in the field of; (a) N,N'-dicarboxymethyl -1,3-propanediamine, which has the formula ##STR1## is sometimes called 1,3-propanediamino-N,N'-diacetic acid and which is designated PDDA; and (b) N,N'-dicarboxymethyl-2-hydroxy-1,3-propanediamine which has the formula ##STR2## which is sometimes called 2-hydroxy-1,3-propanediamine-N,N'-diacetic acid and which is designated HYDROXY-PDDA.
More particularly, this invention is directed to an improved process for preparing PDDA of high quality by an improved route involving the following sequential reactions: ##STR3##
This invention is also directed to HYDROXY-PDDA and to a process for preparing HYDROXY-PDDA of high quality by the following sequential reactions: ##STR4##
Prior art methods of preparing PDDA are taught by Johnson et al., J. Org. Chem. 1962, 27, 2077-2080. See the second column on page 2079 and the first column on page 2080.
The preparation of ##STR5## is taught by Titherly et al, J. Chem. Soc., 1913, 103, 330-340 (at 334).
The preparation of ##STR6## which, elsewhere in this specification, is referred to as ##STR7## from formaldehyde and 1,3-propanediamine is taught by Krassig, Makromol. Chem., 1956, 17, 77-130 (at 87-88).
The process of the instant invention constitutes a decided improvement over prior art routes to PDDA. Among the advantages of the process of the instant invention are; (a) the fact that PDDA formed by the method of this invention is free of side products; (b) the by-products (NH.sub.3, formaldehyde, and an alkali metal salt, e.g., sodium or potassium chloride or sulfate) are readily separated from the respective intermediate or final product with which the by-product is formed; (c) the use of objectional or inconvenient materials such as anhydrous HCl and hydrogenation catalysts is avoided; and (d) the final product is substantially pure PDDA which is obtained without resorting to the expensive and inconvenient repeated decantations and crystallizations of the prior art.